Foam-honeycomb article and method

ABSTRACT

A surface-reinforced lightweight article is described, which includes a honeycomb structure whose sidewalls are reinforced by an expanded foam. The article can be manufactured by laying a fibrous third layer, such as several layers of dry tissue paper backed by a layer of dry fiberglass cloth, on the walls of a mold, laying a second layer of multi-cell honeycomb-like material against the third layer, and laying a finely porous first layer such as tissue paper over the second layer. Foamable plastic is poured in the mold over the three layers and the mold is closed. As the plastic foams and expands, it forces the three layers into a tight sandwich and permeates the layers to reinforce them. Specifically, expanded foam in the cells of multi-cell honeycomb-type material braces the honeycomb sidewalls against collapse, to greatly increase the strength and rigidness of the article. It is found that the density of foam is much greater in the cells than in the core of the article that lies inside the layers, so that high strength foam lies where it is needed while keeping the core lightweight.

This is a continuation of application Ser. No. 07/103,449, filed Sept.30, 1987, now U.S. Pat. No. 4,797,312.

BACKGROUND OF THE INVENTION

The surface of a foam article can be strengthened by using fibrousmaterial, such as tissue paper backed by fiberglass cloth, at thesurface of the article and impregnating the fibrous material with thesame foam used for the core of the article. My earlier patents U.S. Pat.Nos. 4,510,105 and 4,664,974 describe articles of this type and methodsfor manufacturing it. While the articles described in these patentsprovide relatively high strength at the surface and rigidity in theoverall article, while enabling a good surface finish to be produced,there are many applications where further increase in the surface andoverall article rigidity and strength are required as in surfboards,airplane wings, propellers for wind-energy machines, etc. It would bepossible to stack numerous layers of fiberglass to a thickness ofperhaps 1/4 inch near the surface, to be impregnated with foam, but thiswould add considerably to the weight of the finished article, especiallyfor articles having a thickness of no more than a few inches and whereinopposite faces of the article are both reforced. A foam article whichhad a further reinforcement of the foam-impregnated fibrous layer nearthe surface to increase the surface and overall article rigidity andstrength, while minimally increasing the weight of the article, would beof considerable value.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a method andapparatus are provided which result in a foam article having especiallyhigh rigidity and strength at low weight. The article includes afoam-impregnated layer near the surface, backed up by a foam-impregnatedmulti-cell honeycomb-type material. An entire article can include a coreof foam separated from the multi-cell honeycomb type material by aporous but flow-through resistant material, with the foam in themulti-cell material being of higher density than the foam in the core.

The method can include laying a dry fibrous third layer against the moldcavity walls, and laying a multi-cell honeycomb-type second layeragainst the third layer. A porous but flow through-resistant first layercan be laid against the multi-cell second layer. A foamable liquid ispoured into the mold and the mold is closed. As the foamable liquidfoams it expands, presses the layers into a firm sandwich and slowlypenetrates the layers. The foam in the multi-cell second layer can be"trapped" between the first and third layers and can achieve a higherdensity than the foam core to firmly support the sidewalls of themulti-cell honeycomb-type material. The first layer can be fibrous andinclude fine fibers near the surface and coarser fibers further from thesurface. The sidewalls of the multi-cell honeycomb-type materialincludes largely flat sidewall extending perpendicular to the surface ofthe article to provide high rigidity. The expanded foam within the cellsprevent the sidewalls from collapse and also provide continuity betweenthe fiber-impregnated foam region and a possibly unreinforced foam core,to support the fiber-impregnated-foam near the surface and regionsbetween the walls of the multi-cell material.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an open mold useful in the presentinvention.

FIG. 2 is a view taken on the line 2--2 FIG. 1, with the mold closed anda molded product in the mold.

FIG. 3 is an enlarged view of a portion of the molded product of FIG. 2.

FIG. 4 is a partial view taken on the line 4--4 of FIG. 3.

FIG. 5 is an enlarged view of the area 5--5 of FIG. 3.

FIG. 6 is a partial sectional view of an article constructed inaccordance with another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a mold 10 with two parts 12, 14 that form a moldcavity 16 defining the shape of the particular article such as asurfboard, airplane wing, etc. To form the article, a group of layers ofporous dry material shown at 18 in FIG. 2, which will strengthen thesurface region of the finished article, are laid on each mold part. Themixed components of a foamable material are then poured into the cavityportion of one mold part such as 12, and the mold is closed. The foamingmaterial expands to substantially fill the cavity 16, to fill the volume16a lying within the group of surface layers 18 as well as to fill mostof the thickness of the layers. After the foam has hardened the mold isopened, the molded article 20 is removed, and then finishing operationsare performed on the article.

FIG. 3 shows a portion of the article 20 after it is removed from themold. The article includes a thick foam core 22 having opposite sides24, 26, the article having surface regions 30, 32 lying at the oppositesides of the core. Each surface region such as 32 includes a group oflayers 18 impregnated with a foam surface portion 34. A group of layers18 includes a thin porous first layer 36 lying at one side 26 of thefoam core, a multi-cell honeycomb layer or honeycomb type sheetstructure lying on the outer side of the first layer 36, and a fibrousthird layer 42 lying on the outer side of the honeycomb sheet structure40, all three layers being impregnated with foam. A honeycomb type sheetstructure is a structure with interconnecting sidewalls that are each ofa length L (FIG. 5) that is a plurality of times greater than itsthickness T, and that leaves spaces or cells 54 between the sidewallswhich are wider than the sidewall thickness.

FIG. 5 shows details of the region 5--5 FIG. 3. The porous andpreferably fibrous third layer 42 may be formed of multiple sheets oftissue paper 44 at the surface 46 of the unfinished article, backed by awoven fiberglass 50. An additional flow-through resistant but poroussheet 51 of tissue lies behind the fiberglass to help press thefiberglass against the mold wall during molding. The honeycomb 40includes multiple sidewalls 52 extending like columns perpendicular tothe surface 46 of the article, and having a column length L which is aplurality of times greater than the thickness T of each sidewall. Thehoneycomb forms multiple cells 54 that each have a thickness equal tothe height of the sidewalls 52. The first layer 36 is formed of amaterial which is porous to the flow through of foam while resistingfree flow therethrough, and may be formed of a tissue paper, similar tothose at 44 and 51. All of the three layers 36, 40, 42 are impregnatedwith foam of the same type as the foam of the core 22, and which isintegral therewith. However, the foam surface portion 34, including thefoam portion within the honeycomb cells 54, is of a higher density thanthe foam portion 56 lying in the core 22.

The article can be formed by laying sheets of fine fibrous material,such as the multiple tissue sheets 54 on the mold walls, and then amaterial with thicker fibers such as a fiberglass 50, backed by a sheetof fine fibrous material such as tissue sheet 51. The honeycomb sheet 40is then layed on the first layer 42, and the porous first layer 36 islaid against the inner surface of the fiberglass. While the honeycombmaterial is thicker than the other layers, it is very flexible and canbe laid over large regions of the mold, although there can initially beair gaps between the various layers in between the third layer and themold wall. After the layers are emplaced, the foamable mixture is pouredin and the mold is closed, and the foamable material expands within thecavity. As the foamable material expands, it presses up against thesomewhat porous first layer 36 which presses against the honeycomb layer40, which presses against the fibrous third layer 42, to press alllayers into a tight sandwich and the sandwich tightly against the wallsof the cavity. The foam penetrates the first layer, fills the cells 54of the honeycomb layer, defoams and penetrates the tissue sheet 51 andpenetrates the spaces between strands of the fiberglass 50 of the thirdlayer, and slowly penetrates the fine fibrous tissue paper 44 of thethird layer. The amount of foam is such that not all sheets of thetissue paper 44 are penetrated, which facilitate removal of the articlefrom the mold. The manner of laying down the third layer and the actionby which it is pressed against the mode walls and penetrated, is largelysimilar to that described in my earlier U.S. Pat. No. 4,510,105.

It is well known that an article formed of a foam material has theadvantage of very light weight, although without surface reinforcementthe article is subject to damage either at its surface or by damagethrough its entire cross section. In my earlier U.S. Pat. Nos. 4,510,105and 4,664,974, I describe the reinforcement of the surface of a foamarticle by fibrous material impregnated by the same foam as the core ofthe article, with the fibrous material including fine fibers at thesurface backed by coarser fibers just below it, similar to the thirdlayer 42 of FIG. 5. While such foam-impregnated fibrous surfacestructure greatly increases the strength of the article, the increase islimited because the fibrous material is thin such as about 1/64th inchthick. It would be possible to lay many layers of fiberglass against theoutermost layer, to a thickness such as 1/4th inch, which will beimpregnated with foam, but this has several disadvantages. Onedisadvantage is the cost of many layers of fiberglass while anotherdisadvantage is the considerable weight of such a thickness offiberglass. The use a honeycomb layer enables reinforcement of thefibrous layers with a very light weight structure.

The honeycomb layer can have a configuration such as shown in FIG. 4,wherein multiple strips 60 of resin-impregnated paper are bonded intomultiple cells that may be of generally hexagonal shape, although avariety of other shapes can be used. The sidewalls act astension-resisting members, which resist tension forces as indicated at62. The foam within the cells helps resist elongation of the cells, toresist such tension forces. The foam within the cells as well as thecell walls also resist compression forces in the direction of arrows 62.As shown in FIG. 5, the length L of each sidewall 52 of the honeycombtype structure is a plurality of times greater than the thickness T ofthe sidewall. This allows the cell wall to resist bending forces like atall but thin beam and also to resist compression forces like a column.The presence of compressed foam 34 within the cells, which pressesagainst the sidewalls 52 of the honeycomb structure, results in supportfor the sidewalls 52 against collapse, to greatly strengthen thestructure. Since most of the volume of the honeycomb structure in thearticle is occupied by foam, the weight of the foam-filled honeycombstructure is small.

Applicant has found that the above-described method for manufacturingthe foam article results in the foam in the surface portion being ofgreater density than the foam in the core, such as three times as great.This is advantageous because a higher density, and consequently greaterstrength, of foam in the cells enables such foam to better support thesidewalls of the honeycomb type structure.

In one article applicant has constructed, of the type shown in FIGS. 3and 5, the article had an overall thickness A of 21/2 inches. The foamin the core 22 was polyurethane foam which had a density of about 2 lbs.per cubic foot while the foam in the core had a density about threetimes as great. The porous first layer 36 was formed of tissue withopenings of less than 0.01 inch width, which was of low cost andprovided resistence to flowthrough of foam. The third layer 42 includedabout six layers of tissue paper 44, a layer 50 of woven fiberglass of aweight of four ounces per square yard, and a tissue layer 51. Thefiberglass 50 had a thickness of about seven thousanths inch. Thehoneycomb layer 40 had a thickness of 1/4th inch and a weight of 20grams per square foot. It can be seen that it would require perhapsthirty layers of fiberglass to form the same thickness of about 1/4thinch as the honeycomb layer, and the fiberglass layers would weigh aboutthirteen times as much. Also, while the honeycomb layer 40 costs about20 cents per square foot, the fiberglass layer 50 costs about 33 centsper square foot, and about $10.00 per square foot for multiple layershaving a combined thickness of about 1/4th inch. Thus, the use of ahoneycomb-type sheet to back up the fibrous material near the surface,with both being impregnated with foam that is integral with the foam ofthe core, and especially with the foam in the honeycomb cells being of ahigher density than the foam in the core, results in an article of highstrength and low weight, and which can be constructed at low cost.

It may be noted that, as shown in FIG. 3, the first layer 36 has acatenary curvature as at 70 between the sidewalls of the honeycomb sheetstructure 40. This is believed due to the weight of the foaming materialwhen it is laid on the first layer 36. A somewhat similar, though lesspronounced, upward curvature indicated at 72 is also found in the uppersurface region which lies uppermost in the mold.

FIG. 6 illustrates another article 80 wherein a single honeycomb sheetstructure 82 extends along most of the thickness of the article. This isgenerally practical only for an article with parallel opposite surfaces,since an article with nonparallel surfaces such as 20 (FIG. 2) wouldrequire the thickness of the honeycomb layer to vary with articlethickness. The article 80 has porous layers 84, 86 which may be of thetype shown at 42 in FIG. 5, and which lie at the opposite surfaces ofthe article. The article can be molded by placing the lowermost fibrouslayer 86 on a lower mold part, placing the honeycomb sheet structure 82on it, filling each of the cells of a honeycomb with a foamable mixture,placing the uppermost layer 84 on top of the honeycomb structure, andplacing an uppermost mold portion or press on top of the sandwich.

Thus, the invention provides a primarily foam article and method formanufacturing it, which has a highly reinforced surface. Porous(preferably fibrous) material lies close to the surface, and is bracedby honeycomb, with both the porous material and honeycomb material beingimpregnated with foam. This can be accomplished in an article withopposite surface regions each having their own porous (preferablyfibrous) layer and honeycomb layer, with the inner surface of eachhoneycomb layer backed by a porous layer that allows foam to flowtherethrough while resisting such passage, to assure that the layers arepressed firmly against the mold surface. The density of foam in thesurface region, especially in the cells of a honeycomb-type structure,is preferably higher than the density of foam in the core.

Although particular embodiments of the invention have been described andillustrated herein, it is recognized that modifications and variationsmay readily occur to those skilled in the art, and consequently, it isintended that the claims be interpreted to cover such modifications andequivalents.

What is claimed is:
 1. A surface-reinforced article, comprising:a coreof foam material having a first outer side; a surface region of saidarticle which includes a multi-cell honeycomb-type sheet structure lyingon said first outer side of said core and a porous but flow-throughresistant layer lying outside said honeycomb-type structure; said cellsof said honeycomb-type sheet structure, and said flow-through resistantlayer each being impregnated with the same foam material as said core,the foam material in said core, cells, and layer being integral, and thefoam in said cells being at least as dense as the foam in said core. 2.The article described in claim 1, wherein:the foam in said cells has ahigher density than the foam in said core.
 3. The article described inclaim 1, wherein:said core is thicker than said surface region, saidcore is formed only of the foam material which impregnates said cellsand layer, and said core has a second side; and including a secondsurface region substantially identical to said first mentioned surfaceregion, lying at said second side of said core.
 4. The article describedin claim 1, including:an inner layer which is porous but flow-throughresistant lying between said core outer side and said honeycomb-typesheet structure.
 5. A method for forming a reinforced articlecomprising:establishing a porous but flow-through resistant layer at theinside of a mold wall of a mold; establishing a sheet of multi-cellhoneycomb material against a side of said layer which is opposite saidmold wall; placing a foamable material on a side of said sheet ofhoneycomb material which is opposite said mold wall, and closing saidmold to flow said foamable material through said cells of said sheet ofhoneycomb material and at least partially through said layer, andsolidifying said foam material.
 6. The method described in claim 5including:establishing an inner layer of porous but flow-throughresistant material on a side of said sheet of honeycomb material whichis opposite said first mentioned layer, and said step of placing saidfoamable material includes placing it on a side of said inner layerwhich is opposite said sheet of honeycomb material.